On Mon, May 25, 2020 at 12:41:32PM +0100, Mark Brown wrote: > On Sat, May 23, 2020 at 11:34:10AM +0300, Serge Semin wrote: > > On Fri, May 22, 2020 at 04:22:41PM +0100, Mark Brown wrote: > > > > Right, that definitely needs to be fixed then - 8MHz is indeed a totally > > > normal clock rate for SPI so people will hit it. I guess if there's a > > > noticable performance hit to defer to thread then we could implement > > > both and look at how long the delay is going to be to decide which to > > > use, that's annoyingly complicated though so if the overhead is small > > > enough we could just not bother. > > > As I suggested before we can implement a solution without performance drop. > > Just wait for the DMA completion locally in the dw_spi_dma_transfer() > > method and > > return 0 instead of 1 from the transfer_one() callback. In that function > > we'll > > wait while DMA finishes its business, after that we can check the Tx/Rx FIFO > > emptiness and wait for the data to be completely transferred with delays or > > sleeps or whatever. > > No extra context switches there at least, that's the main issue.
Right. There won't be extra context switch. > > > NOTE Currently the DW APB SSI driver doesn't set xfer->effective_speed_hz, > > though as > > far as I can see that field exists there to be initialized by the SPI > > controller > > driver, right? If so, strange it isn't done in any SPI drivers... > > Yes. Not that many people are concerned about the exact timing it turns > out, the work that was being used for never fully made it upstream. > > > What do think about this? > > Sure. Great. I'll send a new patchset soon. It'll fix the Tx/Rx non-empty issue in accordance with the proposed design. -Sergey > > > patchset "spi: dw: Add generic DW DMA controller support" (it's being under > > review in this email thread) ? Anyway, if the fixup is getting to be that > > complicated, will it have to be backported to another stable kernels? > > No, if it's too invasive it shouldn't be (though the stable people might > decide they want it anyway these days :/ ).
